Technique: Gravity Filtration

What is a Drying Agent? How Does it Work?

A drying agent (such as anhydrous Na₂SO₄ or MgSO₄) is a chemical compound that can form hydrates to water. When you perform a liquid-liquid extraction, you eventually isolate an organic liquid phase. Since no organic solvent is 100% perfectly immiscible with an aqueous  phase, you will generally have a small amount of water suspended in the organic phase. This is not good, as we want the organic phase to be as contaminant-free as possible. We need to find a way to remove this water from our organic layer without introducing something that could lead to additional contaminants.

This is where the anhydrous Na₂SO₄ helps us out. By adding in a SMALL amount of Na₂SO₄, we can dry the organic phase. Drying an organic phase means to remove water from the organic phase. It does not mean you are removing the liquid from the organic phase. This means that you have added a solid to the liquid organic phase to complex to the water and form insoluble hydrates with the water. These hydrates (i.e., 9H₂O∙Na₂SO₄) can be removed very easily through gravity filtration. In doing so, we remove both the water, and the drying agent, as the water is now in a non-covalent bonding interaction with the drying agent.

Important notes:

1. The most commonly asked question in any organic chemistry teaching lab is "Is this enough drying agent?"
    

   1. Usually, students add way too much drying agent. The total amount you usually need is usually about one scoop of a spatula. Another way to think of this: the last joint of your pinkie finger.  

   2. If you're adding too much drying agent, you are reducing your percent yield or percent recovery. Why? The solvent (containing your analyte) is going to adsorb (not absorb) to the drying agent's surface, get stuck, and you will lose compound/material.  

2. What to look for to see if you have enough drying agent:
    

   1. Add some drying agent. Let the sample sit for a couple of minutes.  

   2. Confidently swirl the Erlenmeyer flask on the floor of your fumehood. Do not remove the sample from your fumehood - we do NOT want to breathe it!
       

   3. Suddenly stop swirling the flask and look at the drying agent.  

      1. Is none of the drying agent moving, and is it just sitting on the bottom in one large, solid clump? If so, you don't have enough drying agent. Add a little bit more, and repeat the swirling. You should not add more than 5-10% of the total volume equivalent of solvent (i.e., if you have 20 mL of solvent present, adding more than 2 g of drying agent is probably too much and very unnecessary and wasteful. Don't waste the chemicals, that's bad for the environment!  

      2. Is some of the drying agent stuck together in a clump on the bottom, but some of the drying agent flowing around on the bottom of the flask like white sand in a bucket of water? If so, you have enough drying agent. Do not add more, it will negatively impact your experiment.
          

      3. Is all of the drying agent flowing around on the bottom of the flask like white sand in a bucket of water? If so, you have enough drying agent. Do not add more, it will negatively impact your experiment.
         

   4. Another thing you can look for is the clarity of the solution. Sometimes, a "wet" (water-contaminated) organic phase will look a little bit cloudy. If that is the case, the organic phase will quite often become clear upon drying with a drying agent. The water that is contaminating the organic phase is originally suspended in the organic phase as billions of micro-droplets that are bending and refracting the light as it passes through. Once this water is removed by the drying agent, there in nothing left to refract the light, so the solution becomes much less foggy/cloudy.
       

By the end of Chem 08LA, you should never need to ask your TA "Is this enough drying agent?" Get that question out of your system at the start of the quarter. If you are asking this at the end of 08LA, that means you have not been taking our feedback and applying it to your learning. The best way to grow in this lab course and learn is to take our feedback, think about it, apply it, and suddenly watch as things start to work out better and make more sense!

If you are asking this question in Chem 08LB or 08LC, this is a significant issue. We want you to carry over your knowledge from 08LA into these other courses, as 08LB and 08LC build upon the concepts covered in 08LA.

General Guidelines

1. Tare (preweigh) the flask that will be used to collect the liquid component if that contains the desired compound. Choose a flask size that is at least double (ideally triple) the volume of the amount of liquid you intend to introduce. For example, when filtering 25 mL of a mixture, use a 100 mL round bottom flask (RBF).  

   1. This is because the next step after gravity filtration is often rotary evaporation. An RBF should never be placed on the rotovap if it is more than 50% full as it is likely to bump.  

2. Secure a glass funnel on a ring clamp over top of the round bottom flask. Add a small piece of Kimwipe or cotton to plug the stem. If the plug is too big or compressed in the stem, the liquid will not drain through the stem.  

3. Pour the contents through the funnel and let gravity do the work. Depending on how much liquid you plan to filter, you may need to pour some, let it pass through, then pour some more.  

4. Once your have finished your gravity filtration, visually inspect your liquid in your RBF. If you can visibly see solids present, you should gravity filter a second time.  

   1. The solids you see are most likely anhydrous Na₂SO₄, indicating that your cotton/Kimwipe plug was either too loose or it had a hole in it.  

   2. Seeing solids is very rare after gravity filtering. This probably only happens about 5% of the time.